In recent years, a laminate-packaged electric storage device (e.g., battery or capacitor) has been used as a power supply for a portable system, an electric vehicle, and the like. The laminate-packaged electric storage device has a configuration in which an electric storage device element (e.g., battery element) and an electrolyte solution are held inside an outer package that is formed by two outer package films, the electric storage device element being formed by winding or alternately stacking positive plates and negative plates via separators.
The laminate-packaged electric storage device has a problem in that an inflammable gas or the like is generated inside the outer package when the electrolyte solution is electrochemically decomposed or thermally decomposed due to overcharging or exposure to a high temperature, so that the pressure inside the outer package increases. A laminate-packaged electric storage device provided with a safety mechanism has been proposed in order to solve the above problem. The safety mechanism may have a configuration in which the bonding section of the two outer package films of the outer package includes a weakly bonded area, and the weakly bonded area functions as a safety valve for discharging gas when the internal gas pressure has increased, or may be provided with a safety valve that automatically opens when the internal pressure has increased to a value equal to or larger than a given value, and discharges an inflammable gas, for example (see Patent Documents 1 to 5, for example, Patent Document 1: Japanese Patent No. 3554155, Patent Document 2: JP-A-05-013061, Patent Document 3: JP-A-11-086823, Patent Document 4: JP-A-2006-236605, Patent Document 5: JP-A-2007-157678).
FIG. 15 is an exploded view illustrating the configuration of a laminate-packaged electric storage device that is provided with a safety mechanism in which the bonding area of the outer package includes a weakly bonded area. The outer package of a laminate-packaged electric storage device 50 illustrated in FIG. 15 is formed by stacking and heat-sealing the outer edge area of an upper outer package film 51A and the outer edge area of a lower outer package film 51B (i.e., forming a bonding section 52) so that a receiving section that holds an electric storage device element is formed inside the outer package. A thin electric storage device element 55 (e.g., battery element or capacitor element) is provided inside the receiving section of the outer package together with an organic electrolyte solution.
The laminate-packaged electric storage device 50 is configured so that the bonding section 52 includes a weakly bonded area 53. The weakly bonded area 53 functions as a safety valve, so that gas is discharged through the weakly bonded area 53 (i.e., pressure is released) when a large amount of gas is generated inside the outer package. This makes it possible to prevent a situation in which the outer package bursts. More specifically, the weakly bonded area 53 has a seal strength lower than that of the remaining area of the bonding section 52, and separation preferentially occurs in the weakly bonded area 53 to form a vent when the pressure inside the receiving section of the outer package that holds the electric storage device element (battery element or capacitor element) has reached a given value.
In the example illustrated in FIG. 15, the outer package has a rectangular shape, and a positive electrode power supply tab 56 (i.e., common positive electrode lead member) that is electrically connected to a plurality of positive plates included in the electric storage device element 55 (battery element or capacitor element), and a negative electrode power supply tab 57 (i.e., common negative electrode lead member) that is electrically connected to a plurality of negative plates included in the electric storage device element 55, are provided to outwardly extend from either short side of the outer package.
The laminate-packaged electric storage device having the above configuration is required to ensure that separation reliably occurs in the weakly bonded area that forms the safety mechanism to form a vent when the pressure inside the receiving section (space) has reached a given value, and the weakly bonded area has sufficient seal strength so that the outer package is reliably sealed (i.e., exhibits high reliability). However, it is difficult to form a weakly bonded area that has sufficient seal strength from the viewpoint of the production process.
Patent Document 6 (Japanese Patent No. 3859645) discloses a laminate-packaged electric storage device provided with a safety mechanism in which at least one non-bonding section (pressure concentration section) is formed in the area of the bonding section so that the non-bonding section protrudes into the receiving section that holds the electric storage device element, and a pressure release section that connects the inner space and the outer space upon separation of the outer package films is formed in the area of the non-bonding section.
According to the above safety mechanism, however, since stress is relatively uniformly applied over the entire outer edge area of the pressure release section when the internal pressure has increased, separation of the outer package films occurs at an unspecified position of the pressure release section, and leakage of the electrolyte solution from the receiving section may occur depending on the separation position. Moreover, since separation of the outer package films may occur in the pressure release section even if the pressure inside the outer package is low, it is difficult to ensure the seal-tightness of the outer package.